Setting upload speed of SQM slows down download speed


Setting the Upload speed of SQM to 2750 kbit/s for whatever reason brings my download speed to 100 mbps when it should be around 800-900 mbps. I am using cake with the piece_of_cake.qos script, though if I remember right it happened with other options. Any reason this is happening and or any fixes if logs are needed i'll post them but I don't have them at the moment.

Upload parameters are for the download speed. It is the value to send to device. It is in kbps. If you want to reach 900mbps set to 1,000,000.

For TCP (Remo), you expect around 1/40* of the forward traffic as reverse ACK traffic:
100/40 =2.5
so my interpretation is that your forward traffic is rate limited by the achievable ACK rate.
For ~1000 Mbps download traffic you would need >=25 Mbps upload capacity....

*) this assumes forward data segment size around 1460 Bytes and a delayed ACK strategie where one ACK is sent for every two full MSS data segments received, witout delayed ACKs the ACK traffic would need 1/20 of the forward rate. I also note that more modern TCPs and technology like GRO/GSO can reduce the ACK rate, but as a rule of thumb 1/40 is still decent....

If your hands are forced due to having too little upload capacity you could try to enable cake's ack-filter which can help by thinning the ACK stream a bit